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An artist's impression of the Hadean Eon

Earth's four great eons

The history of Earth is divided into four great eons. Each eon saw the most significant changes in Earth's composition, climate, and life. Each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs.

Published 26 October 2019 Main image by Tim Bertelink

4.54 billion years ago — 4.00 billion years ago

Hadean

Hadean (from Hades, the Greek god of the underworld, and the underworld itself) describes the hellish conditions then prevailing on Earth. The planet had just formed and was still very hot owing to its recent accretion (the coming together and cohesion of matter under the influence of gravity), the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies. Source

4.00 billion years ago — 2.50 billion years ago

Archean

Archean (or Archaean) comes from the ancient Greek Αρχή (Arkhē), meaning 'beginning, origin'. During the Archean, the atmosphere was composed of volcanic and greenhouse gases, the Earth's crust had cooled enough to allow the formation of continents, and life started to form. Source

2.50 billion years ago — 541 million years ago

Proterozoic

The name of this eon means 'early life'. Some forms of multicellular organisms emerge, and bacteria begin producing oxygen. Plants, later animals and possibly earlier forms of fungi form around this time. The early and late phases of this eon may have undergone 'Snowball Earth' periods, in which all of the planet suffered below-zero temperatures. Source

541 million years ago — Present

Phanerozoic

Complex life, including vertebrates, begin to dominate the Earth's ocean in a process known as the Cambrian explosion. Gradually, life expands to land and familiar forms of plants, animals and fungi begin appearing, including annelids (segmented worms), insects, and reptiles, hence the eon's name, which means 'visible life'. Modern animals—including humans—evolve at the most recent phases of this eon. Source

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